In recent years, the importance of selection and rapid development of materials has increased, for the purposes of improving performance of products and speeding up their development time.
Taking polymer electrolyte fuel cells (PEFCs) as an example, catalyst layers and gas-diffusion layers are joined in sequence onto both outer surfaces of electrolyte membranes in which power-generation reactions occur, thereby forming membrane/electrode assemblies. Then, the membrane/electrode assemblies are each placed between separators to produce single cells, and a required number of the multiple single cells are stacked to produce polymer electrolyte fuel cells.
A fuel gas containing hydrogen, and an oxidant gas containing oxygen (e.g., the air) are supplied to such polymer electrolyte fuel cells, and thus, the fuel gas and the oxidant gas are electrochemically reacted with each other through the electrolyte membranes. As a result, the polymer electrolyte fuel cells simultaneously produce electric power, heat and water.
A reaction represented by Formula 1 and a reaction represented by Formula 2 occur in negative electrodes and positive electrodes, respectively.H2→2H++2e−  (Formula 1)½ O2+2H++2e−→H2O  (Formula 2)
Hydrogen ions (H+; protons) produced through the reaction in the negative electrodes move inside the electrolyte membranes, and are consumed in the reaction in the positive electrodes. Such electrode reactions occur in the catalyst layers. The catalyst layers are configured by catalyst-supported carbon materials and electrolytes, and thus, the electrode reactions proceed in boundary faces between the catalysts supported onto the carbon materials and the electrolytes. Furthermore, connections between the carbon materials within the catalyst layers serve as pathways for electrons, and connections between the electrolytes serve as pathways for hydrogen ions. In addition, platinum would often be employed for the catalysts supported for both, the positive and negative electrodes. Platinum-based alloy catalysts may also be employed to improve the activities of the positive electrodes and the resistance of the negative electrodes to CO.
Thus, the performance of the catalysts is directly linked to performance of fuel cells. Furthermore, metal materials used for the catalysts (e.g., platinum) are very expensive, and therefore, it would be important to determine their performance at an early stage of the development and by use of smaller amounts.
In view of the above-mentioned issues, measurements of physical properties of the materials are very important, and it is desired that the measurements are carried out by using catalyst powder forms of the materials, i.e., in states that are similar to forms of the materials to be used.
With regards to a method for measuring a resistance of a powdery material, a method in which DC voltages are applied to powdery materials, electric currents generated in that case are measured, and then, resistance values are obtained based on Ohm's law has generally been employed. However, an AC impedance method has been proposed as a more-highly-accurate measurement method (JP-A-2016-27316).
In the AC impedance method disclosed in JP-A-2016-27316, a pair of electrodes are placed at the upper and lower side of a powdery material that is a test subject, and then, an AC voltage or current is applied (input) to the electrodes while varying the frequency. Impedances are calculated based on output currents or voltages corresponding to the input AC voltages or currents. Relationships between frequencies of the input signals and the impedances are computed, the Nyquist plot is conducted, and a shape of the Nyquist plot is subjected to equivalent circuit fitting. According to these procedures, it becomes possible to measure resistance components accurately.
FIG. 8 is a schematic view that describes the conventional method for measuring a resistance of a powdery material. Arrangements among the components required in the powder resistance-measurement method in FIG. 8 will described below.
A measurement vessel 51 is configured by side-wall sections 52, a bottom 57, and a cover 58. The bottom 57 is configured by a bottom insulation part 57a, a bottom electrode 57b, and a bottom terminal 54. The cover 58 is configured by a cover insulation part 58a, a cover electrode 58b, and a cover terminal 53.
A powdery material 59 that is a measurement sample is put into an internal space 56, and is placed between the bottom electrode 57b and the cover electrode 58b. In a state in which a pressing device 60 applies pressure to the measurement sample 59, impedances are measured between the cover terminal 53 and the bottom terminal 54.